Essentially all primary aluminum production takes place in electrolytic cells commonly referred to as Hall-Heroult cells, after the American and French inventors of the process. In these cells, aluminum oxide is dissolved in a bath of molten cryolite, maintained at a temperature of approximately 1,000.degree. C. The aluminum oxide is reduced to metallic aluminum by electric current passing through the bath from carbon anodes suspended in the bath to carbon cathodes that line the bottom of the cells. In the process, the carbon anodes are gradually consumed.
When in operation, these cells are operated 24 hours a day, seven days a week, because of the cost of starting and stopping operation. However, the cells must be shut down periodically to repair or replace the cathodes, to adjust production to meet demand, or for various other reasons.
When a Hall-Heroult cell is started, or restarted, the cell is typically heated by passing electrical current through the cell for 14-24 hours. During this period the cathode temperature is brought from ambient temperature to a temperature of 600.degree.-700.degree.. It is desirable to heat the cathodes, and other cell components, gradually and uniformly over this period to avoid thermal shock to and undue stresses on the cathode blocks from alternating hot and cold areas of the cathode created by uneven distribution of the 10,000 or more KWH applied to heat it.
The life of a cell, i.e. the length of time the cell can be operated before it must be rebuilt again, depends in part on how well the cell is baked-out and started. The life of the individual cell can be reduced by up to 25% by poor bake-out and start-up procedures. With the cost of rebuilding a cell approaching $100,000.00, the need for proper bake-out procedures is obvious.
Increasing cell life also reduces the amount of spent potlining that is generated. Disposal of these wastes is another significant cost.
There is no aluminum or bath in the cell to complete the electric circuit during the pre-heat cycle, and the anode blocks are commonly spaced from the cathodes by resistive carbon containing material between the anode blocks and the cathode. The anodes, cathode, and other components of the cell expand as they are heated. The rods that support the anodes (an individual cell may contain 24 anodes) do not all take current at exactly the same rate, and therefore do not all heat and expand at the same rate. With the rods clamped tightly against the anode bus, in accordance with conventional practice, as one or more rods grow and push up on the cell superstructure, they can and do lift adjacent rods off the resistive material between the anodes and cathode. This accentuates the differences in anode support rod expansion, and creates cool and hot spots in the cathode. The resultant thermal stress may cause cracking of cathode blocks and premature failure of the cell.
In addition, as the cathode begins to heat up, it tends to arch up in the center and middle while each end of the cathode actually moves down. The corner and end anodes can actually be held off the resistive material, while the center or middle anodes push down with the weight of the entire superstructure against the middle cathode blocks, held in place by tamping paste which is now in a plastic state. These forces and stress on the cathode can cause premature cracking and even result in metal infiltration of the cathode, reducing the operating efficiency and shortening the life of the pot or cell.
In a typical prior art cell utilizing 24 anodes per cell, each supported by an anode support rod clamped to a contact button on the anode bus, clamps may be loosened and retightened every hour during the 14-24 hours of the pre-heat cycle. A standard procedure calls for a worker to loosen the clamps on 12 anodes on one side of the cell with a pneumatic impact wrench, retighten all 12, and then repeat the procedure for the 12 anodes on the other side of the cell. This physically demanding work requires an extra worker for each pot that is being pre-heated. The voltage readings on the anode support rods frequently indicate that, in the middle or toward the end of the pre-heat cycle, the anodes actually need to be loosened and retightened more frequently then once per hour.
Systems for supporting anodes in a Hall-Heroult cells so that the anodes can be adjusted individually have been proposed, e.g. in U.S. Pat. No. 4,394,242 to Clark, U.S. Pat. No. 4,414,070 to Spence, and U.S. Pat. No. 4,465,578 to Duclaux et al. Most of these systems are expensive, however, and none provide a mechanism for adjusting the position of a group of anodes in response to thermal expansion of cell components during a pre-heat cycle.